This invention relates to an electrophotographic apparatus, such as printers including laser printers and electrophotographic copiers, and particularly, relates to an electrophotographic apparatus having a device for fixing a toner image with stability on a recording medium and an electrophotographic method.
In a conventional electrophotographic apparatus, as shown in Japanese patent Laid-open print No. 57-828773, etc., a fixing apparatus is formed as a duplex cylinder in which an outer portion is formed of a material with a large modulus of elasticity and an inner portion is formed of a material with a higher heat conductivity than, the outer portion, and a heating roller having a heater near the central axis thereof and a compression roller to pressurize the recording medium against the heating roller, and fixes a toner image on the recording medium by passing it between the rollers.
Japanese patent Laid-open print No. 4-178679 discloses a method of determining the temperature at the time of fixing by detecting an increased temperature of the heating component when a main power of the fixing roller is on.
Japanese patent Laid-open print No. 2-161481 discloses an apparatus wherein a heating temperature is changed according to an output of a temperature sensor.
Japanese patent Laid-open print No. 63-172187 discloses an apparatus wherein temperature modes are set in advance and the temperature control modes are changed over time with a prescribed temperature.
Japanese patent Laid-open print No. 5-281875 discloses an apparatus wherein a fixing temperature is changed according to the kind of recording paper and whether color printing or black-and-white printing is specified by a person. The fixing equipment uses a rubber material with low heat conductivity for the roller surface. Therefore, a time delay occurs until heat given by the heating means arrives at the roller surface. When a recording medium passes between the rollers of the fixing device, the temperature of the roller surface drops rapidly. When the temperature drop of the roller surface is detected, and when the heating means is turned on to provide heat, the roller surface temperature decreases gradually because of the time delay mentioned above. When the heat detection means detects the specified control temperature, and when the heating means is turned off, the temperature on the roller surface becomes too high (i.e. overshoot) because the quantity of heat released has been excessive.
Particularly, at the time of starting up the fixing device or at the time of the start of the first page from the waiting state, the temperature change of the fixing device is large until the temperature becomes stable or the fixing temperature is not constant when the continuous printing operation is performed; thus stable image quality cannot be guranteed.
Japanese patent Laid-open print No. 4-178679 discloses a technology by which a set value for the fixing temperature is determined by the start-up temperature of the fixing device. This technology, however, can not solve the problem due to change of environment. Japanese patent Laid-open print No. 2-161481 discloses a counter measurement to the environmental change. However, the problems such as an excess temperature rise by the temperature detection delay occur.
An object of this invention is to provide an electrophotographic apparatus with a fixing device which can maintain stable image quality regardless of the temperature change and time passage.
The present invention provides an electrophotographic apparatus having a first means for detecting the state of a fixing device before its start-up and a second means for detecting the state of the fixing device during fixing operation. The fixing voltage control means has a means for changing the control point of fixing voltage according to the temperature change and time passage by information or data of on the state of the fixing device or by the fixing voltage control characteristics.
The temperature compensation means compensates a drop of the fixing roller surface temperature per every page in such a manner that if the kind of the recording medium and the color image are fixed, the temperature decrease of the roller is changed passed on a thickness of toner. The temperature decrease by the kind of the recording medium and the frequency of the number of superposition of the color toner images is detected, then a heating means is turned on by extracting the time corresponding to the optimum heat quantity from the table stored in memory in advance.
In addition, the upper and lower limits of temperature compensation per every page are monitored. Further, temperature compensation time of every page is conducted if too low temperature is detected by monitoring the upper limit of temperature compensation of every page and means of monitoring the lower limit of temperature compensation of every page. An input quantity of heat and a taken quantity of heat are balanced by adding correction for compensation time extracted by using the temperature compensation means for every page.
The fixing voltage control means changes the control point according to the temperatures change and time passage by using the temperature characteristic of the surface temperature of the fixing roller. Therefore, the optimum fixing voltage can always be applied to the fixing roller, so that stabilization of image quality can be expected. The unstable state of the temperature at the time of starting up of the fixing device is detected before start-up. The fixing voltage control means changes the control point according to the temperature change and time passage by using the temperature characteristic of the surface temperature of the fixing roller, so that the optimum fixing voltage can be applied to the fixing roller at the time of start-up, and image quality can be stabilized.
The heat taken by the recording medium from the fixing roller is compensated by the temperature compensation means for every page; the fixing temperature that is stabilized at the time of continuous printing can be kept constant; and image quality can be maintained.